Lymphocyte Activation Gene-3, or LAG-3 (also known as CD223), is a member of the immunoglobulin supergene family, and is expressed on activated T cells (Huard et al. (1994) Immunogenetics 39:213), NK cells (Triebel et al. (1990) J. Exp. Med. 171:1393-1405), regulatory T cells (Huang et al. (2004) Immunity 21:503-513; Camisaschi et al. (2010) J Immunol. 184:6545-6551; Gagliani et al. (2013) Nat Med 19:739-746), and plasmacytoid dendritic cells (DCs) (Workman et al. (2009) J Immunol 182:1885-1891). LAG-3 is a membrane protein encoded by a gene located on chromosome 12, and is structurally and genetically related to CD4.
Similar to CD4, LAG-3 can interact with MHC class II molecules on the cell surface (Baixeras et al. (1992) J. Exp. Med. 176:327-337; Huard et al. (1996) Eur. J. Immunol. 26:1180-1186). It has been suggested that the direct binding of LAG-3 to MHC class II plays a role in down-regulating antigen-dependent stimulation of CD4+ T lymphocytes (Huard et al. (1994) Eur. J. Immunol. 24:3216-3221) and LAG-3 blockade has also been shown to reinvigorate CD8+ lymphocytes in both tumor or self-antigen (Gross et al. (2007) J Clin Invest. 117:3383-3392) and viral models (Blackburn et al. (2009) Nat. Immunol. 10:29-37). Further, the intra-cytoplasmic region of LAG-3 can interact with LAP (LAG-3-associated protein), which is a signal transduction molecule involved in the downregulation of the CD3/TCR activation pathway (Iouzalen et al. (2001) Eur. J. Immunol. 31:2885-2891). Moreover, CD4+CD25+ regulatory T cells (Treg) have been shown to express LAG-3 upon activation, which contributes to the suppressor activity of Treg cells (Huang, C. et al. (2004) Immunity 21:503-513). LAG-3 can also negatively regulate T cell homeostasis by Treg cells in both T cell-dependent and independent mechanisms (Workman, C. J. and Vignali, D. A. (2005) J. Immunol. 174:688-695).
Given the importance of LAG-3 in downregulating an immune response, the need exists for developing novel agents that modulate its activity to activate the immune system. Such agents can be used, e.g., for cancer immunotherapy and treatment of other conditions, such as chronic infection.